As used herein, "mineral wool" includes all man-made vitreous fibers made from rock, stone and slag which are variously referred to as rock wool and slag wool. Mineral wool fibers are typically formed by melting mineral-laden rocks and materials, such as basaltic rock, limestone, dolomite and metallurgical slags, and attenuating the molten mass into fibers by spinning or other techniques. While the types and amounts of the various components of the melt depend largely on the requisite melt-processing characteristics and the properties desired for the resulting fibers, basalt is usually the primary raw material. Commonly, a flux such as dolomite, limestone or slag is added to reduce the melting temperature of the mixture and improve its processability into fibers.
The compositions of mineral wool fibers vary depending on the intended end-use application. Exemplary rock and slag wool compositions, with amounts given in weight percentages, are as follows:
______________________________________ Rock Wool Rock Wool Slag Wool made from made from made from basalt melted basalt and other slag melted Component in a furnace in a cupola in a cupola ______________________________________ SiO.sub.2 45-48% 41-53% 38-52% CaO 10-12% 10-25% 20-43% MgO 8-10% 6-16% 4-14% Al.sub.2 O.sub.3 12-13.5% 6-14% 5-15% K.sub.2 O 0.8-2% 0.5-2% 0.3-2% Na.sub.2 O 2.5-3.3% 1.1-3.5% 0-1% TiO.sub.2 2.5-3% 0.9-3.5% 0.3-1% FeO* 11-12 3-8 0-2 S 0-0.2 0-0.2 0-2 P.sub.2 O.sub.5 0-0.5 ______________________________________ *: In rock and slag wool produced from materials in a cupola with coke as fuel, substantially all iron oxide is reduced to FeO. During the spinning process, a surface layer may form in which the iron is oxidized to Fe.sub.2 O.sub.3. Typically 8-15% of the iron is oxidized to Fe.sub.2 O.sub.3. In an electric furnace melting basalt, up to 50% of the iron is in the form of Fe.sub.2 O.sub.3 and is more evenly distributed throughout the entire fiber volume.
In view of the possibility that such fibers, or fragments thereof, can be inhaled, a desire exists for the development of mineral fiber compositions that exhibit a high biosolubility, while preferably maintaining acceptable processability and good insulating and fire-retarding characteristics. The solubility of mineral wool fibers in human lung fluid is affected by the chemical composition of the fibers. In particular, the alumina content of the fiber has been found to have a significant impact on the solubility of the fiber. Although mineral wool fibers typically contain from 4 to 16% alumina (unless indicated otherwise, all percentages and ratios given herein are by weight), enhanced solubility has been reported to be exhibited by fibers containing amounts of alumina outside this range. For example, low alumina-content fibers having high biosolubility are disclosed in International Patent Publication No. WO 94/14717, and high alumina-content fibers having high biosolubility are discussed in V. R. Christensen et al., "Effect of Chemical Composition of Man-made Vitreous Fibers on the Rate of Dissolution In Vitro at Different pHs," Environmental Health Perspectives, 102 (Suppl.5), 83-86 (1994).
Mineral wool fibers containing at least 18% alumina also can have improved high-temperature stability. For example, mineral wool fibers containing 18-25% alumina and having high softening temperatures are disclosed in U.S. Pat. No. 4,560,606, and mineral wool fibers containing 31-38% alumina that exhibit improved resistance to high temperatures are disclosed in U.S. Pat. No. 5,312,806. High alumina-content mineral wool fibers are particularly desirable for high-temperature applications.
While low alumina-content compositions can be obtained by the selective use of suitable rock as the main raw material, compositions with low amounts of alumina are difficult to fiberize effectively due to rapid crystallization and viscosity control problems. In contrast, compositions with greater than 18% alumina require the addition of minerals or chemical agents to augment the alumina content of the standard raw materials. This is particularly true if the ratio of alumina to silica in the composition exceeds about 0.35. Although alumina powder, china clay or calcined bauxite chippings can be used for this purpose, these materials all have the disadvantages of being relatively expensive and requiring consolidation into blocks or briquettes of suitable size before they can be used in a cupola furnace, which adds further to the cost. Accordingly, a need exists for an economical means to introduce sufficient alumina into the melt to obtain good processability and increased high-temperature stability in the resulting fibers. This need has now been met by the our discovery that uncalcined lumps of raw bauxite may be added to mineral wool compositions melted in a cupola furnace to provide mineral wool fibers exhibiting increased resistance to softening at high temperatures.